The present invention relates to an improved photoreceptor belt system for an electrostatic reproduction machine.
In conventional xerography, a xerographic plate (photoreceptor) comprising a layer of photosensitive insulating material affixed to a conductive backing is used to support electrostatic latent images. In the xerographic process, the photosensitive surface is electrostatically charged, and the charged surface is then exposed to a light pattern of the image being reproduced to thereby discharge the surface in the areas where light strikes the surface. The undischarged areas of the surface thus form an electrostatic charge pattern (an electrostatic latent image) conforming to the orginal pattern. The latent image is then developed by contacting it with a finely divided electrostatically attractable powder referred to as "toner". Toner is held on the image areas by the electrostatic charge on the surface. Where the charge is greater, a greater amount of toner is deposited. Thus, a toner image is produced in conformity with a light image of the copy being reproduced. Generally, the developed image is then transferred to a suitable transfer member (e.g., paper), and the image is affixed thereto to form a permanent record of the original document.
In the practice of xerography, the transfer member is caused to move in synchronized contact with the photosensitive surface during the transfer operation, and an electrical potential opposite from the polarity of the toner is applied to the side of the paper remote from the photosensitive surface to electrostatically attract the toner image from the surface to the paper.
In a modern high-speed electrostatic reproduction machine, the photoreceptor is in the form of an endless belt, and the belt is mounted over a plurality of rollers for movement around a closed path. During its movement, the belt is held against a flat perforated plate by a vacuum, there being a platen cloth stretched over the plate so as to prevent direct contact between the belt and plate. It was noted by the Applicant that during high volume runs on such a machine, two machine problems consistently delayed or terminated such runs. These two problems were false paper jams, and the photoreceptor being scratched during machine operation. During investigation, the Applicant eventually discovered that both of these problems were caused by the same thing -- dirty or contaminated platen cloths. These prior art platen cloths are made of densely woven white nylon, and they become contaminated with over-sprayed interface material (polycarbonate and polyurethane) and toner which may require that the cloths be changed after as few as 60K copies are made. The photoreceptor life is normally greater than 120K copies. Thus, not only do contaminated platen cloths cause the above two problems, but they further increase the risk of scratching or otherwise damaging the photoreceptors while removing the dirty cloths and installing clean cloths since the photoreceptors must be removed and handled in order to change the cloths.
The applicant found that both the false paper jams and the scratching of the photoreceptor during machine operation were due to buckling of the photoreceptor belt, and that the buckling was caused by excessive contamination of the platen cloth which increased the coefficient of friction between the back of the belt and the cloth. A contaminated platen cloth has a larger surface area of contact than an uncontaminated cloth, and consequently the coefficient of friction between a photoreceptor belt and the former will be larger than between the photoreceptor belt and the latter. A main drive roller "pulls" the photoreceptor belt from an idler roller and "pushes" the belt toward and across a cleaning station where residual toner is cleaned from the photoreceptor belt. As the photoreceptor belt is "pushed" toward the cleaning station, the belt passes a detecting means or sensor located just upstream of the cleaning station and adjacent to the belt. Normally, this sensor senses when a copy has not been stripped from the photoreceptor, causes the machine to stop and indicates that there is a paper jam. The sensor includes a light source and a light receiver, and distinguishes paper as a change in reflectance. As the photoreceptor belt is "pushed" toward the sensor, and as the platen cloth becomes increasingly contaminated, a wave or " buckle" forms in the photoreceptor belt which causes a false mis-strip jam; the buckle causes a change in the coefficient of reflection which is sensed as a mis-strip by the sensor. When the buckle in the photoreceptor belt becomes severe enough, it actually contacts the sensor and scratches the belt.
During his investigation, the Applicant also discovered that prior art platen cloths appear to be contributing to or creating another long standing problem namely, "pink-banding" (crystallization of the photosensitive layer). Crystallization of the photosensitive surface reduces the effectiveness thereof and consequently results in lowering copy quality. The applicant discovered that photoreceptor belts used with prior art platen cloths show a marked correlation between the "pink-band" patterns on the photosensitive surfaces of the belts, and the scumming or streaking patterns on the backs of the belts. Thus, it appears that contamination of prior art platen cloths with toner increases friction between the photoreceptor belts and the cloths which increases the heat in local "bands" which results in fusing toner to the back of the photoreceptor belt which results in local crystallization or "pink-banding" of the photosensitive surface of the photoreceptor belt. Contamination of the platen cloth also decreases the vacuum applied to the back of the belt which reduces the heat sink and causes the belt to get even hotter.